This invention relates to improvements in a Leonard type elevator system.
In reversible Leonard type elevator systems a decrease in the source voltage and the failure of the power supply when the converter receives regenerative power from an associated DC motor leads to a fear that the thyristors cannot turn off and thereby permitting rush currents to flow thereinto resulting in their destruction. In order to avoid this feared result, it is a common practice to provide on Leonard systems an undervoltage detector designed and constructed so that it responds to an AC voltage less than a predetermined magnitude to interrupt the main circuit on the DC side. If the output voltage from the Leonard type elevator system is low, a more or less decrease in AC voltage has caused the undervoltage detector to interrupt the main circuit although the converter is able to controllably drive the DC motor connected thereto. Particularly, in such Leonard systems used with the elevator system this interruption of the main circuit has resulted in the emergency stoppage of the elevator car involved. This has shocked passengers in the elevator car causing them uneasiness. Therefore it is desirable to make the number of emergency stops as small as possible.
Accordingly it is an object of the present invention to eliminate the objections to the prior art practice as above described by the provision of a new and improved Leonard type elevator system for stably operating an elevator car involved without the emergency stoppage of the elevator car each time the AC voltage supplied to the system decreases.